The literature has long recognized that the switched reluctance (SR) motor is fault tolerant because it can continue operating and produce torque with one or more of its phases not functioning. For example, in U.S. Pat. No. 4,896,089 to Kliman et al, where various means are described to identify faults, the corrective action upon detecting a fault is “that the faulted motor phases are deactivated” (column 9, lines 49–50). See also “Fault Detection and Management System for Fault-Tolerant Switched Reluctance Motor Drives, by Charles M. Stephens, IEEE Transactions on Industry Applications, Vol. 27, No. 6, Nov/Dec 1991. The focus in the prior art, then, has been to construct SR drives with separate inverters, power supplies and electronics for each phase, thus allowing an easy disconnect of any particular phase, as in U.S. Pat. No. 5,708,576 to Jones et al.
In “Fault Tolerant Operation of Single-Phase Generators” by T. Sawata, P. C. Kjaer, C. Cossar, and T. J. E. Miller, published in IEEE Transactions on Industrial Applications, vol. 35, No. 4, July/August 1999, the authors discuss the fact that a particular phase may be made of several windings. If these windings are separately excited, one can remove excitation from one winding and continue operating the rest of that phase. A similar idea is proposed in. U.S. Pat. No. 5,517,102 to Jahns, where 2 inverters are used for each phase and, in case of a failure, one half of a phase is disconnected (that is, excitation to that phase is removed) while the other half continues working and provides half of the nominal torque from that phase. In U.S. Pat. No. 6,020,711 to Rubertus et al, an SR drive is shown where the various phases and poles can be reconfigured to minimize the impact of removing a faulty circuit. In all 3 cases cited in this paragraph (Sawata et al, Jahns and Rubertus et al), though, the faulty circuit is removed.
In U.S. Pat. No. 5,737,164 to Ferreira et al, the authors point out that, when an SR motor is designed such that each phase has 4 poles, then turning off only 2 poles can be damaging because the flux from the poles remaining in the on state will travel through the poles with the damaged winding. Their proposed solution is to turn off all windings aligned with a rotor pole associated with a fault. Therefore, they go beyond the previous prior art and disable not only the damaged windings, but other windings as well.
A particular problem occurs with a faulty SR motor at standstill. The torque produced by an SR machine is produced by each phase sequentially. Therefore, if one phase is fully disabled (disconnected, etc), there is no torque available for some angular positions, spanning an angle somewhat less than 360°/Nph Nr, where Nph and Nr are the numbers of phases and rotor poles, respectively. Disabling a phase completely is an issue during starting if the rotor is in the wrong angular position.
This starting problem has also been recognized and addressed in the prior art. The authors of U.S. Pat. No. 4,896,089, cited above, point out that if a motor cannot move clockwise as desired, due to an open phase, then it is possible to first move the motor counterclockwise to some other position, from which it can be excited to move clockwise. In some applications, a small rotation in the wrong direction may be acceptable, but not so in others.
U.S. Pat. No. 5,517,102 to Jensen teaches that, when specially designed, some SR motors do not have any dead zone for starting. However, the design conditions are severely limiting; for instance, there must be a minimum of 5 phases, etc.
U.S. Pat. No. 4,896,088, cited above, suggests using two separate excitation circuits (inverters, etc) for each phase. For instance, each excites two separate windings. Then, in case of a failure, only one half of the phase is removed and, concerning starting, half the starting torque is still available in the “dead zone”. This solution, however, is costly, and relies again on disabling the faulty circuits.